The search for optimum production conditions for a fissured reservoir depends on having a good description of the fissure pattern. Hence the sizes and volumes of the matrix blocks must be defined at all points in a structure. However, the geometry of the medium (juxtaposition and shapes of blocks) in usually too complex for such computation. This is why, in a previous paper, we got around this problem by reasoning on the bases of averages (clips, azimuths, fissure spacing), and thot led us to an order of magnitude of the volumes. Yet a mean volume cannot be used to explain the distribution of block volumes. But it is precisely this distribution that qoverns the choice of one or several successive recovery methods. Therefore, this article describes an original method for statistically computing the distribution law for matrix-block volumes. This method con be applied of any point in a reservoir. The reservoir portion involved with blocks having a given volume is deduced from this method. A general understanding of the fracturing phenomenon acts as the basis for the model. Subsurface observations on reservoir fracturing provide the data (histogrom of fracture direction and spacing). An application to the Eschau field (Alsace, France) is described here to illustrate the method.